Searching for Signs of Life in the Reflected Light from Exoplanets: A Catalog of Nearby Target Stars

Abstract

Which stars are the best stars to search for habitable planets and signs of life? This is a trick question, because it depends not only on the kind of circumstellar environment we think is likely to be supportive to life as we know it, but it depends also on the technique being used to do the search. For example, the Catalog of Nearby Habitable Stellar Systems was designed for SETI, a search for technological signals. Because this search strategy relies on life forms out-shining their star (at least at certain frequencies), target selection is not complicated by the need to spatially resolve the habitable planets on which these life forms presumably live. On the other hand, because the life forms being sought are technologically advanced, it seems reasonable to assume that their planet had to be continuously habitable for long enough to evolve such biological complexity. Thus the deciding factor for SETI is that of long term habitability. Meanwhile, other missions to directly detect habitable planets (e.g., NASA’s TPF and ESA’s Darwin) are less worried about long term habitability but must struggle with the competing factors of planet separation from the star and planet brightness relative to the star. This paper outlines a variety of challenges in the search for simple and complex life in the Solar Neighborhood.